Businessman David Perdue won the Georgia Senate Republican primary runoff race, advancing to the general election where he will square off against Democrat Michelle Nunn in one of the marquee races of the 2014 Midterm Elections that could tilt control of the upper chamber.

Mr. Perdue, former CEO of Dollar General and Reebok, cast himself as a Washington outsider. He was declared the winner roughly four hours after polls closed, narrowly defeating Rep. Jack Kingston — an 11-term congressman who was endorsed by the the U.S. Chamber of Commerce.

“I’ve never run for anything in my life,” Mr. Perdue told supporters at his victory night party. “I’m humbled.”

Voters also headed to the polls to cast ballots in three House runoff races for seats that Mr. Kingston and fellow Republican Reps. Paul C. Broun and Phil Gingrey gave up to seek the GOP nomination for Senate.

Former state Sen. Barry Loudermilk, a tea party favorite, cut former Georgia Rep. Bob Barr’s political comeback short, handily defeating him in the race for the GOP nod in the 11th Congressional District.

The winners of the three GOP primary races are expected to capture the seats in the November election.

But the outcome of the general election race to replace retiring Sen. Saxby Chambliss is more in doubt, thanks to Mrs. Nunn — daughter of former Georgia Sen. Sam Nunn — who, to the delight of Democrats, stepped down from her leadership post at George H.W. Bush’s Points of Light Foundation to seek the seat.

Since then, Mrs. Nunn easily won the Democratic nomination and has emerged as a fundraising powerhouse. Over the last three months, she raised $3.45 million, besting both of her possible GOP rivals. Mr. Perdue, though, has shown he is willing to put millions into his campaign.

Libertarian Amanda Swafford also is running in the general election.

Jennifer Duffy of the Cook Political Report said the general election race is a toss-up.

“The reality is that Nunn has gotten a big head start on money and organization,” Ms. Duffy said. “She has been a candidate for a year and she hasn’t gotten a whole lot of criticism, whereas the Republican is going to have picked up some negatives from the primary and the runoff race. Whoever is going to be the Republican nominee will have some catching up to do.”